A router is a device that forwards data packets between computer networks, creating an overlay internetwork. A data packet is forwarded from one router to another through the networks that constitute the internetwork until it gets to its destination node. Routers on the internet may control traffic. The act of dropping packets to “slow” down flows is called shaping and policing.
Most modern routers use queues to segregate traffic. The router may use upon the queue a number of Advanced Queue Management (“AQM”) techniques. One of the existing AQM techniques to control traffic is called a drop tail technique. This technique involves dropping all incoming packets when a queue becomes full. When the packets exit the queue and the queue size lowers, then incoming packets are admitted. Another one of the AQM techniques is called a Random Early Detection (“RED”) technique. According to the RED technique, a queue threshold is set at two levels. When the queue reaches the first threshold, the packets are randomly dropped. Once the queue reaches the second threshold then all incoming packets are dropped, as in the drop tail technique.
According to congestion controlled protocols, such as Transmission Control Protocol (“TCP”) or Stream Control Transmission Protocol (“SCTP”), when a sender sees drops of packets the sender slows down a packet sending rate. There are different ways according which the sender can slow down the packet sending rate. According to one way, if the sender determines that few packets are missing, the sender performs a fast re-transmit of these packets and reduces the sending rate of the packets to ½ that of what it was previously transmitting at. According to other way, when the sender determines that many packets are missing, the sender takes a timeout and stops sending all packets. After taking the timeout the sender enters a slow start by sending only one packet first, then two packets, and so on to slowly work back up from the full stop timeout.
For the above-mentioned AQM techniques, the routers have no knowledge or awareness of the traffic that they are dropping. There is a random chance of a timeout to occur. For example, if multiple packets are dropped in succession in a congestion controlled flow, such as Transmission Control Protocol (“TCP”) flow or Stream Control Transmission Protocol (“SCTP”) flow, then the odds increase greatly that the sender will stop sending the packets in the flow altogether and will take a full scale timeout, reverting the flow's congestion window to one Maximum Transmission Unit (“MTU”). Generally, MTU is defined as the size of the largest packet that a network protocol can transmit. The flow then will enter slow-start (as defined by RFC2581 “TCP Congestion Control” document). The full stop of the flow unfairly penalizes and harms the one particular flow while other flows get more bandwidth then the one that took a full stop timeout.